This invention relates to processes for chemically-modifying the surface of emulsion polymer particles and to surface-modified emulsion polymer particles. More particularly, it relates to a process for chemically-modifying the surface of emulsion polymer particles which provides increased surface concentration of bound surface-modifying chemical on the particles relative to the surface concentration which would be provided by prior processes.
It is often desirable to have a particular composition or functionality, referred to herein as xe2x80x9csurface-modifying chemicalxe2x80x9d, chemically bound to the surface of a polymer. For example, it is desirable to have acid-functionality attached to the surface of emulsion polymer particles to assist in the stabilization of the polymer particles in water. This may be effected, to varying degrees, by polymerizing acid-containing monomer throughout the polymerization of the other monomers, during portions of the polymerization of the other monomers, as a part of the polymerization of the monomers used to form an outer stage of a multi-stage polymer, or some combination thereof. However, incorporating the acid-containing monomer in parts of the emulsion polymer particle where it is not needed or desired is not only expensive and wasteful but may also cause performance problems such as reduced water-resistance and corrosion resistance and increased water sensitivity. In addition, attempts to provide bound surface-modifying chemical in a later or outer stage often leaves unincorporated or residual surface-modifying chemical in the aqueous medium. This residual surface-modifying chemical may further contribute to the water sensitivity of the system.
Even when one is successful in accomplishing complete incorporation of the surface-modifying chemical in an outer stage on the polymer particle, the polymer chains forming the outer stage may actually penetrate into the inner stage(s) of the polymer rather than forming a discrete outer stage. This penetration of the polymer chains forming the outer stage buries some surface-modifying chemical that is a part of the polymer chains within the polymer particle rather than on its surface. If the surface-modifying chemical is hydrophilic in nature or otherwise will form hydrogen bonds with water (as is the case with copolymerized acid-functional monomers), then it is believed that the polymer particle would be more thermodynamically stable if the material of interest were at the interface between the water and polymer particle surface in aqueous systems. However, because of the nature of the polymer chain entanglements, the polymer chains that contain the surface-modifying chemical may lack the necessary mobility to move to the interface between the water and polymer particle surface. Thus, it would be desirable to provide a process to improve the mobility of the polymer chains containing the bound surface-modifying chemical within the polymer particles, control the location of the surface-modifying chemical exclusively to where it is desired and thereby improve the overall efficiency of action of the material of interest.
A number of different techniques have been employed to improve the mobility of the polymer chains but each technique has serious drawbacks. For example, the polymer particles may be heated, preferably to a temperature above the glass transition temperature of the outer stage polymer, to improve the mobility of the polymer chains. However, heating involves an extra step and may degrade the polymer particles. Alternatively, the polymer particles may be permitted to age for a sufficient period of time to permit the polymer chains to move to the most thermodynamically stable configuration. This alternative is not acceptable because the time required for such movement could be unreasonably long, perhaps months or even years. A third option is to add an organic solvent to soften to the outer stage of the polymer particle and permit greater mobility of the polymer chains. This option leaves a solvent in the composition that may pose attendant safety, health and environmental problems and may need to be removed.
EP 915,108 A discloses a process for producing multistage emulsion polymers with improved extent of swelling and maintenance of dry bulking density, including the steps of:
(a) providing an aqueous emulsion of
(i) multistage emulsion polymer, containing a core stage polymer and a shell stage polymer, wherein the core stage polymer contains, as polymerized units:
(1) 5% to 100% by weight, based on the weight of the core stage polymer, of hydrophilic monoethylenically unsaturated monomer, and
(2) 0% to 95% by weight, based on the weight of the core stage polymer, of at least one nonionic monoethylenically unsaturated monomer; and
wherein the shell stage polymer contains, as polymerized units at least 50% by weight, based on the weight of the shell stage polymer, of nonionic monoethylenically unsaturated monomer; and
(ii) monomer at a level of at least 0.5% by weight based on the weight of the multistage emulsion polymer; and
(iii) swelling agent; under conditions wherein there is no substantial polymerization of the monomer; and
(b) reducing the level of monomer by at least 50%.
EP 959,176 A discloses processes to control the location of a second shell polymer in a multistaged polymer particle. The second shell polymer is formed from 93% to 99.9% by weight, based on the total weight of the second shell polymer, of at least one nonionic monoethylenically unsaturated monomer and from 0.1% to 7% by weight, based on the total weight of the second shell polymer, of an acid-functional monoethylenically unsaturated monomer. The second shell polymer has a glass transition temperature from xe2x88x9215xc2x0 C. to xe2x88x9250xc2x0 C.
Applicant has discovered a process to produce polymer particles having a chemically-modified surface. By employing this process, applicant produces polymer particles where the surface-modifying chemical need not be incorporated in portions of the polymer particles where it is not needed or where it is detrimental.
According to a first aspect of the present invention there is provided a process for chemically-modifying the surface of a polymer particle, including the steps of: (1) providing an aqueous emulsion comprising: (a) a multistage emulsion polymer including a core stage polymer and a shell stage polymer, wherein the core stage polymer includes, as polymerized units, from 5% to 100% by weight, based on the weight of the core stage polymer, of hydrophilic monoethylenically unsaturated monomer, and from 0% to 95% by weight, based on the weight of the core stage polymer, of at least one nonionic monoethylenically unsaturated monomer; and wherein the shell stage polymer includes, as polymerized units, at least 50% by weight of nonionic monoethylenically unsaturated monomer; (ii) a monomer at a level of at least 0.5% by weight based on the weight of the multistage emulsion polymer; and (iii) at least one surface-modifying chemical capable of chemically bonding with the monomer; under conditions wherein there is no substantial polymerization of the monomer; and (2) reducing the level of the monomer by at least 50%.
According to a second aspect of the present invention there is provided a process for chemically-modifying the surface of a polymer particle, including the steps of: (a) providing an aqueous emulsion, including: (i) a multistage emulsion polymer including a core stage polymer and a shell stage polymer; wherein the core stage polymer includes, as polymerized units, 0.1% to 100% by weight, based on the weight of the core stage polymer, of at least one nonionic monoethylenically unsaturated monomer; and wherein the shell stage polymer contains, as polymerized units: (1) at least 50% by weight, based on the weight of the shell stage polymer, of nonionic monoethylenically unsaturated monomer; and (2) at least one surface-modifying monomer; (ii) a monomer at a level of at least 0.5% by weight based on the weight of the multistage emulsion polymer; and (iii) neutralizing agent; under conditions wherein there is no substantial polymerization of the monomer; and (b) reducing the level of monomer by at least 50%.
According to a third aspect of the present invention there is provided a process for chemically-modifying the surface of a polymer particle including:(a) providing an aqueous emulsion including
(i) an emulsion polymer containing, as polymerized units:
(1) 5% to 100% by weight, based on the weight of said polymer, of hydrophilic monoethylenically unsaturated monomer and (2) 0% to 95% by weight, based on the weight of said polymer, of at least one nonionic monoethylenically unsaturated monomer;
(ii) a monomer at a level of at least 0.5% by weight based on the weight of said emulsion polymer;
(iii) optionally, neutralizing agent; and
(iv) at least one surface-modifying chemical capable of chemically bonding with said monomer;
under conditions wherein there is no substantial polymerization of said monomer; and
(b) reducing the level of said monomer by at least 50%.
The stages of the multistage polymers of the present invention include core stage 0 polymer (the xe2x80x9ccorexe2x80x9d), and shell stage polymer (the xe2x80x9cshellxe2x80x9d). By xe2x80x9ccorexe2x80x9d and xe2x80x9cshellxe2x80x9d herein is not intended to include only particles where a spherical core is completely encapsulated by a shell of uniform thickness. Rather, it is understood that other multistage polymer morphologies such as multiple cores, non-spherical cores, shells of non-uniform thickness, and shells incompletely encapsulating cores are also contemplated. Further, the core and shell may themselves be made up of more than one stage. There may also be one or more intermediate stages. Preferably, the multistage polymer comprises a core, an intermediate layer and a shell.
The cores of the multistage polymers of the present invention are emulsion polymers comprising, as polymerized units, from 5% to 100% by weight, based on the weight of the core, of at least one hydrophilic monoethylenically unsaturated monomer and from 0% to 95% by weight, based on the weight of the core stage polymer, of at least one nonionic monoethylenically unsaturated monomer.
Cores containing at least 5% by weight, based on the total weight of the core polymer, of at least one hydrophilic monoethylenically unsaturated monomer will generally result in a suitable degree of swelling. There may be instances wherein, because of the hydrophobicity of certain comonomers or combinations thereof in conjunction with the hydrophobic/hydrophilic balance of a particular hydrophilic monomer, the copolymer may be suitably prepared with less than 5% by weight, based on the total weight of the core polymer, of a hydrophilic monoethylenically unsaturated monomer. Preferably, the core comprises, as polymerized units, hydrophilic monoethylenically unsaturated monomer at a level of from 5% to 100%, more preferably, from 20% to 60%, and most preferably, from 30% to 50% by weight based on the total weight of the core. The hydrophilic core polymer may be made in a single stage or step of the sequential polymerization or may be made by a plurality of steps in sequence.
The multistage emulsion polymer of the present invention contemplates a core polymer wherein at least one hydrophilic monoethylenically unsaturated monomer is polymerized alone or with at least one nonionic monoethylenically unsaturated monomer. This process also contemplates, and includes in the term xe2x80x9chydrophilic monoethylenically unsaturated monomer,xe2x80x9d the use of a nonpolymeric compound containing at least one carboxylic acid group which may be absorbed into the core polymer before, during or after the polymerization of the hydrophobic shell polymer as a replacement for the hydrophilic monoethylenically unsaturated monomer in the hydrophilic core polymer, as described in U.S. Pat. No. 4,880,842. In addition, this invention contemplates, and includes in the term xe2x80x9chydrophilic monoethylenically unsaturated monomer,xe2x80x9d the use of a latent hydrophilic core polymer which contains no hydrophilic monoethylenically unsaturated monomer but which is swellable upon hydrolysis to a hydrophilic core polymer as described in U.S. Pat. No. 5,157,084.
Suitable hydrophilic monoethylenically unsaturated monomers useful for making the core polymer include, for example, acrylic acid, methacrylic acid, acryloxypropionic acid, methacryloxypropionic acid, itaconic acid, aconitic acid, maleic acid or anhydride, fumaric acid, crotonic acid, monomethyl maleate, monomethyl fumarate, monomethyl itaconate and the like. Acrylic acid and methacrylic acid are preferred.
Suitable nonpolymeric compounds containing at least one carboxylic acid group include C6-C12 aliphatic or aromatic monocarboxylic acids and dicarboxylic acids, such as benzoic acid, m-toluic acid, p-chlorobenzoic acid, o-acetoxybenzoic acid, azelaic acid, sebacic acid, octanoic acid, cyclohexanecarboxylic acid, lauric acid and monobutyl phthalate and the like.
Suitable nonionic monoethylenically unsaturated monomers for making the hydrophilic core polymer include styrene, xcex1-methyl styrene, p-methyl styrene, t-butyl styrene, vinyl toluene, ethylene, vinyl acetate, vinyl chloride, vinylidene chloride, (meth)acrylonitrile, (meth)acrylamide, (C1-C20) alkyl or (C3-C20) alkenyl esters of (meth)acrylic acid, such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, benzyl (meth)acrylate, lauryl (meth)acrylate, oleyl (meth)acrylate, palmityl (meth)acrylate, stearyl (meth)acrylate and the like. By xe2x80x9c(meth)acrylatexe2x80x9d herein is meant acrylate or methacrylate.
The core, whether obtained by a single stage process or a process involving several stages, may have an average particle size of from 50 nm to 1.0 micron, preferably from 100 nm to 300 nm, diameter in unswollen condition. If the core is obtained from a seed polymer, the seed polymer preferably has an average particle size of from 30 nm to 200 nm.
The core may also optionally contain from 0 to 20% by weight, preferably from 0.1 to 3% by weight, based on the total weight of the core, of polyethylenically unsaturated monomer, wherein the amount used is generally approximately directly proportional to the amount of hydrophilic monoethylenically unsaturated monomer used; in other words, as the relative amount of hydrophilic monomer increases, it is acceptable to increase the level of polyethylenically unsaturated monomer. Alternatively, the core polymer may contain from 0.1 to 60% by weight, based on the total weight of the core polymer, of butadiene.
Suitable polyethylenically unsaturated monomers include alkylene glycol, di(meth)acrylates, such as for example, ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1,3-butylene glycol diacrylate, 1,4-butylene glycol diacrylate propylene glycol diacrylate and triethylene glycol dimethylacrylate; 1,3-glycerol dimethacrylate; 1,1,1-trimethylol propane dimethacrylate; 1,1,1-trimethylol ethane diacrylate; pentaerythritol trimethacrylate; 1,2,6-hexane triacrylate; sorbitol pentamethacrylate; methylene bis-acrylamide, methylene bis-methacrylamide, divinyl benzene, vinyl methacrylate, vinyl crotonate, vinyl acrylate, vinyl acetylene, trivinyl benzene, triallyl cyanurate, divinyl acetylene, divinyl ethane, divinyl sulfide, divinyl ether, divinyl sulfone, diallyl cyanamide, ethylene glycol divinyl ether, diallyl phthalate, divinyl dimethyl silane, glycerol trivinyl ether, divinyl adipate; dicyclopentenyl (meth)acrylates; dicyclopentenyloxy (meth)acrylates; unsaturated esters of glycol monodicyclopentenyl ethers; allyl esters of xcex1,xcex2-unsaturated mono- and dicarboxylic acids having terminal ethylenic unsaturation including allyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate, diallyl itaconate and the like.
The multistage polymer of the present invention preferably contains an intermediate stage. The intermediate stage polymer, when present, partially or fully encapsulates the core and itself is partially or fully encapsulated by the shell. The intermediate stage is prepared by conducting an emulsion polymerization in the presence of the core.
The intermediate stage preferably contains, as polymerized units, from 0.3% to 20%, more preferably from 0.5% to 10% by weight, based on the weight of the core, of at least one hydrophilic monoethylenically unsaturated monomer. The intermediate stage preferably contains, as polymerized units, from 80% to 99.7%, more preferably from 90% to 99.5% by weight, based on the weight of the intermediate stage, of at least one nonionic monoethylenically unsaturated monomer. The hydrophilic monoethylenically unsaturated monomers and the nonionic monoethylenically unsaturated monomers useful for making the core are also useful for making the intermediate layer.
The shell of the multistaged polymer of this invention is the product of emulsion polymerizing at least 50%, preferably from 80% to 100%, more preferably from 90% to 100%, percent by weight, based on the total weight of the shell, of at least one nonionic monoethylenically unsaturated monomer. The nonionic monoethylenically unsaturated monomers suitable for the core are also suitable for the shell. Styrene is preferred.
The shell may also contain, as polymerized units, from 0% to 20%, preferably from 0% to 10%, by weight based on the weight of the shell, of one or more monoethylenically unsaturated monomers containing acid-functionality including acrylic acid, methacrylic acid, acryloxypropionic acid, (meth)acryloxypropionic acid, itaconic acid, aconitic acid, maleic acid, maleic anhydride, fumaric acid, crotonic acid, monomethyl maleate, monomethyl fumarate, monomethyl itaconate and the like. Acrylic acid and methacrylic acid are preferred.
In embodiments where a voided emulsion polymer particle is desired, the monomers used for the shell polymer and the relative proportions of them in the shell should be such that the shell is permeable to an aqueous or gaseous volatile or fixed basic neutralizing agent capable of swelling the core. In such cases monomeric mixtures for making the shell preferably contain from 0.1% to 10% by weight, based on the total weight of the shell polymer, of an acid-functional monoethylenically unsaturated monomer. Preferably, the proportion of acid-functional monoethylenically unsaturated monomer in the shell polymer does not exceed one-third the amount in the core polymer.
The presence of acid-functional monoethylenically unsaturated monomer in the shell polymer may serve several functions:
(1) stabilizing of the final multistage emulsion polymer;
(2) assuring permeability of the shell to a swelling agent; and
(3) compatibilizing the shell with the previously formed stage of the multistage emulsion polymer.
As used herein, the term xe2x80x9csequentially emulsion polymerizedxe2x80x9d or xe2x80x9csequentially emulsion producedxe2x80x9d refers to polymers (including homopolymers and copolymers) which are prepared in aqueous medium by an emulsion polymerization process in the presence of the dispersed polymer particles of a previously formed emulsion polymer such that the previously formed emulsion polymers are increased in size by deposition thereon of emulsion its polymerized product of one or more successive monomer charges introduced into the medium containing the dispersed particles of the preformed emulsion polymer.
In the sequential emulsion polymerization of the multistage emulsion polymer, the term xe2x80x9cseedxe2x80x9d polymer is used to refer to an aqueous emulsion polymer dispersion which may be the initially-formed dispersion, that is, the product of a single stage of emulsion polymerization or it may be the emulsion polymer dispersion obtained at the end of any subsequent stage except the final stage of the sequential polymerization. Thus, a hydrophilic core polymer which is herein intended to be encapsulated by one or more subsequent stages of emulsion polymerization may itself be termed a seed polymer for the next stage.
The process of this invention contemplates that the core, the intermediate stage, the shell, or any combination thereof may be made in a single stage or step of the sequential polymerization or may be made by a plurality of steps in sequence following the polymerization. The first stage of emulsion polymerization in the process of the present invention may be the preparation of a seed polymer containing small dispersed polymer particles insoluble in the aqueous emulsion polymerization medium. This seed polymer may or may not contain any hydrophilic monomer component but provides the nuclei on which the hydrophilic core polymer, with or without nonionic comonomer, is formed.
The emulsion polymer of the third aspect of the present invention contains, as polymerized units, hydrophilic and nonionic monoethylenically unsaturated monomers; suitable monomers include those exemplified hereinabove for the core polymer of the multistage emulsion polymer.
A water-soluble free radical initiator is utilized in the aqueous emulsion polymerization. Suitable water-soluble free radical initiators include hydrogen peroxide; t-butyl peroxide; t-butyl hydroperoxide; t-amyl hydroperoxide; alkali metal persulfates such as sodium, potassium and lithium persulfate; ammonium persulfate; and mixtures of such initiators with a reducing agent. Reducing agents include: sulfites, such as alkali metal metabisulfite, hydrosulfite, and hyposulfite; sodium formaldehyde sulfoxylate; and reducing sugars such as ascorbic acid and isoascorbic acid. The amount of initiator is preferably from 0.01% to 3% by weight, based on the total amount of monomer and in a redox system the amount of reducing agent is preferably from 0.01% to 3% by weight based on the total amount of monomer. The temperature may be in the range of 10xc2x0 C. to 100xc2x0 C. In the case of the persulfate systems, the temperature is preferably in the range of 60xc2x0 C. to 90xc2x0 C. In the to redox system, the temperature is preferably in the range of 30xc2x0 C. to 70xc2x0 C., preferably below 60xc2x0 C., more preferably in the range of 30xc2x0 C. to 45xc2x0 C. The type and amount of initiator may be the same or different in the various stages of the multistage polymerization.
One or more nonionic or anionic emulsifiers, or surfactants, may be used, either alone or together. Examples of suitable nonionic emulsifiers include tert-octylphenoxyethylpoly(39)-ethoxyethanol, dodecyloxypoly(10)ethoxyethanol, nonylphenoxyethyl-poly(40)ethoxyethanol, polyethylene glycol 2000 monooleate, ethoxylated castor oil, fluorinated alkyl esters and alkoxylates, polyoxyethylene (20) sorbitan monolaurate, sucrose monococoate, di(2-butyl)phenoxypoly(20)ethoxyethanol, hydroxyethylcellulosepolybutyl acrylate graft copolymer, poly(ethylene oxide)/poly(butyl acrylate) block copolymer, block copolymers of propylene oxide and ethylene oxide, 2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylated with 30 moles of ethylene oxide, N-polyoxyethylene(20)lauramide, N-lauryl-N-polyoxyethylene(3)amine and poly(10)ethylene glycol dodecyl thioether. Examples of suitable anionic emulsifiers include sodium lauryl sulfate, sodium dodecylbenzenesulfonate, potassium stearate, sodium dioctyl sulfosuccinate, sodium dodecyldiphenyloxide disulfonate, nonylphenoxyethylpoly(1)ethoxyethyl sulfate ammonium salt, sodium styrene sulfonate, sodium dodecyl allyl sulfosuccinate, linseed oil fatty acid, sodium or ammonium salts of phosphate esters of ethoxylated nonylphenol, sodium octoxynol-3-sulfonate, sodium cocoyl sarcocinate, sodium 1-alkoxy-2-hydroxypropyl sulfonate, sodium alpha-olefin (C14-C16)sulfonate, sulfates of hydroxyalkanols, tetrasodium N-(1,2-dicarboxy ethyl)-N-octadecylsulfosuccinamate, disodium N-octadecylsulfosuccinamate, disodium alkylamido polyethoxy sulfosuccinate, disodium ethoxylated nonylphenol half ester of sulfosuccinic acid and the sodium salt of tert-octylphenoxyethoxypoly(39)ethoxyethyl sulfate. One or more surfactants are generally used at a level of from 0 to 3% by weight, based on the weight of the stage polymer. The surfactant(s) may be added prior to the addition of any monomer charge, during the addition of a monomer charge or a combination thereof. In certain monomer/emulsifier systems for forming the shell, the tendency to produce gum or coagulum in the reaction medium may be reduced or prevented by the addition of 0.05% to 2.0% by weight, based on total weight of the shell polymer, of emulsifier without detriment to the deposition of the polymer formed on the previously formed core particles.
The amount of emulsifier may be zero, in the situation wherein a persulfate initiator is used, to 3% by weight, based on the weight of the multistage polymer. By carrying out the emulsion polymerization while maintaining low levels of emulsifier, the subsequent stages of polymer-formation deposit the most-recently formed polymer on the existing dispersed polymer particles resulting from the preceding step or stage. As a general rule, the amount of emulsifier should be kept below that corresponding to the critical micelle concentration for a particular monomer system, but while this limitation is preferable and produces a unimodal product, it has been found that in some systems the critical micelle concentration of the emulsifier may be exceeded somewhat without the formation of an objectionable or excessive number of dispersed micelles or particles. It is for the purpose of controlling the number of micelles during the various stages of polymerization so that the deposition of the subsequently formed polymer in each stage occurs upon the dispersed micelles or particles formed in the previous stages, that the concentration of emulsifier is kept low.
The weight-average molecular weight of the polymer formed in a given stage, as may be measured using gel permeation chromatography, may range from 100,000, or lower if a chain transfer agent is used, to several million. When 0.1% to 20% by weight, based on the weight of the monomer, of a polyethylenically unsaturated monomer mentioned hereinbefore is used in making the core, the molecular weight is increased whether or not crosslinking occurs. The use of the polyethylenically unsaturated monomer reduces the tendency of the core polymer to dissolve when the multistaged polymer is treated with a swellant for the core. If it is desired to produce a core having a molecular weight in the lower part of the range, such as from 20,000 to 500,000, it is frequently most practical to do so by using no polyethylenically unsaturated monomers and using a chain transfer agent, such as 0.05% to 2% or more by weight based on the weight of the stage polymer, such as, for example, alkyl mercaptans or alkyl mercaptoalkanoates.
The weight ratio of core to the intermediate stage, if present, is generally in the range of from 1:0.5 to 1:10, preferably in the range of from 1:1 to 1:7. The weight ratio of core to shell is generally in the range of from 1:5 to 1:20, preferably in the range of from 1:8 to 1:15.
The amount of polymer deposited to form shell polymer is generally such as to provide an overall diameter of the multistage polymer particle of from 70 nm to 4.5 microns, preferably from 100 nm to 3.5 microns, more preferably from 200 nm to 2.0 microns, in unswollen condition (that is, before any neutralization to raise the pH to at least 6) whether the shell polymer is formed in a single stage or in a plurality of stages. When the hydrophilic core polymer is fully encapsulated, it does not titrate with alkali metal bases under analytical conditions of about 1 hour at room temperature. The extent of encapsulation can be determined by removing samples during the course of the shell polymerization and titrating with sodium hydroxide.
The multistage emulsion polymer is prepared by sequential emulsion polymerization, which, as discussed above, includes charging the monomers which form the shell. At, or near, the conclusion of charging the monomers which form the shell, the contents of the reactor include the multistage polymer, water and unreacted monomer. Under the conditions of an emulsion polymerization, there is also an appreciable free-radical content, or radical flux, which keeps the polymerization process going. Even if no additional monomer or initiator is added, there is an appreciable free-radical content in the system. When there is no appreciable free-radical content, in other words, when the radical flux is very low or approaches zero, then no substantial amount of polymerization will occur.
We have discovered that this free-radical content interferes with the extent of swelling which can be achieved. Previously known processes typically achieve swelling by adding a suitable swelling agent in the latter stages of charging the monomers which form the shell or at the completion of charging the monomers which form the shell. It is believed that the presence of unreacted monomer facilitates the transport of the swelling agent to the core. However, in previously known processes, the swelling agent was added to the system while there was still an appreciable free-radical content in the system. Thus, under those conditions, a substantial amount of polymerization was still occurring.
We have discovered that by providing an aqueous emulsion of the multistage emulsion polymer, monomer and neutralizing agent under conditions wherein there is no substantial polymerization of the monomer, we can enhance the extent of swelling of the multistage emulsion polymer.
There are many methods for providing that no substantial polymerization of monomer is occurring, including the addition of one or more polymerization inhibitors, the addition of one or more reducing agents, waiting for a sufficient period of time until there are no longer an appreciable number of free-radicals by virtue of them terminating, cooling the contents of the reactor to limit the reactivity of the free-radicals, and combinations thereof A preferred means involves the addition of one or more polymerization inhibitors such as, for example, N,N-diethylhydroxylamine, N-nitrosodiphenylamine, 2,4-dinitrophenylhydrazine, p-phenylenediamine, phenathiazine, alloocimene, triethyl phosphite, 4-nitrosophenol, 2-nitrophenol, p-aminophenol, 4-hydroxy-TEMPO (4-hydroxy-2,2,6,6,-tetramethylpiperidinyloxy, free radical), hydroquinone, p-methoxyhydroquinone, tert-butyl-p-hydroquinone, 2,5-di-tert-butyl-phydroquinone, 1,4-naphthalenediol, 4-tert butyl catechol, copper sulfate, copper nitrate, cresol and phenol. When used, the polymerization inhibitors or reducing agents are added in an amount effective to substantially stop any polymerization, generally from 25 to 5,000 parts per million (xe2x80x9cppmxe2x80x9d), preferably from 50 to 3,500 ppm based on polymer solids. Preferably, the polymerization inhibitor(s) or reducing agent(s) are added while the multistage polymer is at or below the temperature at which the shell was polymerized, most preferably less than 10xc2x0 C. below the temperature at which the shell was polymerized.
Monomer which is present after providing that no substantial polymerization of monomer is occurring may be:
(i) one or more of the monomers used to prepare any of the stages of the multistage or single stage polymer;
(ii) one or more monomers other than those used to prepare any of the stages of the multistage or single stage polymer; or
(iii) combinations thereof.
Preferably, monomer present at such time is one or more of the monomers used to prepare the shell. Such monomer may be unreacted monomer from preparing the multistage emulsion polymer, it may be separately added, or a combination thereof Preferably, the monomer is nonionic monomer. Nonionic monomer is preferred because acid-functional monomers will be neutralized by the neutralizing agent, and these neutralized monomers may be difficult to remove by polymerization. Preferably the level of monomer present after providing that no substantial polymerization of monomer is occurring is from 1 to 20 times as much as the standing monomer level during polymerization.
The surface-modifying chemical useful in the process of the invention is any chemical entity that is capable of chemically bonding with the ethylenically unsaturated monomer(s) present at a level of at least 0.5% by weight based on the weight of the emulsion polymer in step (a)(ii) of the processes of Claims 1-3 of the present invention. A wide variety of surface-modifying chemicals may be employed in the process of the invention, including monomers having at least one site of ethylenic unsaturation, terminally-unsaturated oligomers, egg hydroxyethyl cellulose or other cellulosics, polyvinyl alcohol, poly(N-vinyl pyrolidone), and polyacrylamides.
The amount of the surface-modifying chemical employed in the process of the invention depends largely on the final amount of the compound that is desired to be present on the surface of the emulsion polymer particles. Preferably, the surface-modifying chemical may be incorporated at 0.1% to 10% by weight, more preferably 0.2% to7.5% by weight, based on the dry weight of the polymer particles.
It is preferred to use one or more neutralizing agents. Suitable neutralizing agents include, are those which, in the presence of the multistage emulsion polymer and monomer, are capable of permeating the shell and swelling the core. Neutralizing agents may be aqueous or gaseous, volatile or fixed bases or combinations thereof.
Suitable neutralizing agents include volatile bases such as ammonia, ammonium hydroxide, and volatile lower aliphatic amines, such as morpholine, trimethylamine, and triethylamine, and the like; fixed or permanent bases such as potassium hydroxide, lithium hydroxide, zinc ammonium complex, copper ammonium complex, silver ammonium complex, strontium hydroxide, barium hydroxide and the like. Solvents, such as, for example, ethanol, hexanol, octanol, TEXANOL solvent and those described in U.S. Pat. No. 4,594,363, may be added to aid in fixed or permanent base penetration. Ammonia and ammonium hydroxide are preferred.
When trying to maximize the extent of swelling, it is preferable that the one or more neutralizing agents are added after providing that no substantial polymerization of monomer is occurring. The amount of neutralizing agent can be less than, equal to or greater than the amount needed to provide for complete neutralization of the core. Preferably, the amount of neutralizing agent is in the range of from 75% to 300%, more preferably in the range of from 90% to 250% based on the equivalents of the functionality in the core capable of being neutralized. It is also preferable to add the one or more neutralizing agents to the multistage emulsion polymer while the multistage emulsion polymer is at an elevated temperature, preferably at a temperature within 10xc2x0 C. of the shell polymerization temperature. Swelling is generally very efficient under conditions of elevated temperature, in the presence of monomer and no substantial polymerization occurring. Under these conditions, swelling is generally complete within 30 minutes, preferably within 20 minutes, most preferably within 10 minutes of adding the one or more swelling agents.
The core polymer of the multistage emulsion polymer swells when the core is subjected to a basic swelling agent that permeates the shell to at least partially neutralize the hydrophilic-functionality of the core, preferably to a pH of at least about 6, more preferably to a pH of at least about 10, and thereby result in swelling of the hydrophilic core polymer. The swelling, or expansion, of the core may involve partial merging of the outer periphery of the core into the pores of the inner periphery of the shell and also partial enlargement or bulging of the shell and the entire particle overall.
When the swollen multistage emulsion polymer is dried, water and/or neutralizing agent are removed from the central region of the swollen multistage emulsion polymer, the core tends to shrink and a void develops, the extent of which depends upon the resistance of the shell to restoration to its previous size. This resistance of the shell restoring itself to its previous size is critical for minimizing the dry bulk density of the swollen multistage emulsion polymer. The expansion of the core results in expansion of the shell also. As the size of the shell is restored to its previous size, the dry bulk density increases. It is desirable, therefore, to minimize the extent to which the size of the shell is restored, thereby maximizing the dry bulk density of the swollen multistage emulsion polymer.
This can be accomplished by reducing the monomer level. It is believed that the presence of monomer may be helpful in facilitating the swelling of the multistage polymer, whether by plasticizing the shell, aiding in the transport through the shell or a combination thereof. However, the presence of monomer may be detrimental when trying to maximize swelling and minimize the dry bulk density of the swollen multistage emulsion polymer. Accordingly, after swelling the multistage emulsion polymer in the presence of both monomer and swelling agent, it is desirable to reduce the level of monomer to less than 10,000 ppm, preferably to less than 5,000 ppm based on polymer solids. This may be accomplished by any suitable means. Preferably, the level of monomer is reduced by polymerizing the monomer. This may be accomplished by any suitable means, such as by adding one or more initiators such as those recited above. It is preferred to begin to reduce the level of monomer within 20 minutes, more preferably within 10 minutes, of adding the one or more neutralizing agents.